The short gravitational wave detector developed by researchers at the University of Western Australia spotted two strong events during the 153 days of an experiment conducted in 2019. These signals could come from the beginning of the Universe, or be the result of an interaction of dark matter with a black hole. For now, the scientists who detected them cannot interpret them with certainty.
Gravitational waves are huge ripples observed in the fabric of space-time, generated by collisions of black holes and neutron stars in the distant Universe – events so powerful that they send waves through the whole cosmos, at wavelengths of the order of hundreds of kilometers. Facilities such as Laser Interferometer Gravitational-Wave Observatory (LIGO) were designed to detect these specific waves.
However, there are also much shorter gravitational waves, a few meters to a few kilometers long, the origin of which is poorly understood by the scientific community. Therefore, some researchers set out to build detectors specially adapted to search for these smaller, high frequency waves. Among them: Michael Tobar, physicist at the University of Western Australia in Perth and his team; and it turns out that their device recently detected some mysterious signals.
An origin difficult to determine
The high-frequency gravitational wave detector designed by Tobar and his colleagues consists of a quartz crystal disc about three centimeters in diameter, with a resonance chamber that produces an electrical signal whenever it vibrates at certain frequencies. . Tobar compares this configuration to a gong which would emit a particular sound if it were struck by a gravitational wave: the “sound” emitted by the crystal is here picked up as an electromagnetic signal.
To protect their detector from any “parasitic” electromagnetic field, the researchers have equipped their device with several anti-radiation screens. The device was also cooled to very cold temperatures (around 4K) to minimize thermal vibrations. Two events were detected during their experiment in 2019, in which they monitored two shear modes (slow and fast) simultaneously.
The first event occurred on May 12, 2019 and was observed in the 5.506 MHz mode; on the other hand, no event was observed in the second mode at 8.392 MHz. During a second phase of testing, a second event occurred on November 27, 2019 and was observed in both modes (at 5.506 MHz and 4.993 MHz).
It remains to determine the origin of these results. Cosmic radiation – a flow of atomic nuclei and high-energy particles from the interstellar medium – could be a possible explanation, according to Tobar. But the observed signals might as well come from a previously unknown type of thermal fluctuation in the crystal (which should have been minimized by the extremely cold temperatures).
A signal from the early Universe?
In fact, “a multitude of exotic perspectives” could explain the observed signals, according to the researchers. Axions – the hypothetical particles proposed as one of the constituents of dark matter – revolving around a black hole could also emit gravitational waves. Finally, assuming that the origin of the signals is based on a whole new physics, anything is possible: ” Many explanations might require physics previously unknown, beyond the Standard Model which describes almost all subatomic particles and forces in the Universe. Tobar said.
These signals could also come from the far reaches of the Universe, from the very first moments after the Big Bang. Indeed, cosmologists believe that after this event, the Universe experienced a period of exponential expansion, at the end of which it could have undergone a phase transition – much like when liquid water passes through l gaseous state at a certain temperature, explains Francesco Muia, theoretical physicist at the University of Cambridge, who was not involved in this study. However, this transition ” could have deposited large amounts of energy in the fabric of space-time, generating gravitational waves that could be seen by this experiment “, Says Muia.
But before reaching a definitive conclusion, additional evidence must be provided. The researchers hope that other experiments similar to theirs will soon be put online; simultaneous detection of the same signal by several devices could potentially direct scientists to its origin, in particular by excluding the internal processes of the crystal.